I would try to find a way physically to get him to do it rather than trying to get him to conceptualize it. For example, you might try having him hold the bar in the crease of his hips, arms long and relaxed, and tell him to push it back into his hips - if he doesn't get that, pull on the bar and tell him not to let you move it.

While pushing it in like that, have him bend at the knees only slowly, then jump through the heels and move his shoulders backward behind his hips to snatch it. This is about as simple as you can make it, and usually you get a breakthrough with something similar to that.